This is a past event.

Please join this week's Atmospheric Sciences Graduate Student presenter Susan Nathai (Physics) for their in-person presentation Monday, April 10th at 4 PM - Fisher Hall 101.

Optical Properties of Atmospherically Aged Tar Balls

Susan Mathai (Physics) [Advisor: Dr. Claudio Mazzoleni]

Tar balls are spherical brown carbonaceous particles with high viscosity and light-absorption. They are believed to form in biomass burning smoke during transport in the troposphere. Tar balls have a significant impact on the Earth’s radiative balance, but due to poorly characterized optical properties, this impact is highly uncertain. Here, we investigate the chemical composition and optical properties of individual tar balls transported in the free troposphere to the Climate Observatory “Ottavio Vittori” on Mt. Cimone, Italy (2165 meters above sea level) using multi-modal micro-spectroscopy. Our results show that tar balls contributed 50% of carbonaceous particles by number in the size range from 0.25 to 1.8 μm. Of those tar balls, 16% were inhomogeneously mixed with other constituents. Using electron energy loss spectroscopy, we retrieved the complex refractive index from 200 to 1200 nm for both inhomogeneously and homogeneously mixed tar balls. We found the average refractive index of inhomogeneously mixed particles to be 1.40 + 0.03i and the refractive index of homogeneously mixed particles to be 1.36 + 0.03i at 550 nm. Futhermore, we estimated the top of the atmosphere radiative forcing using the Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) and found that a layer of tar balls with an optical depth of 0.1 above vegetation has a positive radiative forcing ranging from 2.8 Wm ^-2 (on a clear sky day) to 9.5 Wm ^-2 (when cloud is below the aerosol layer). Understanding the optical properties of tar balls will help reduce uncertainties associated with the contribution of biomass-burning aerosol in current climate models.